Conventionally, fine irregularities are formed on a surface of a solar battery so as to suppress surface reflection. These fine irregularities cause multiple reflection of incident light and allow the incident light to be efficiently absorbed into the solar battery. These fine irregularities are referred to as “texture”.
Generally, in a monocrystalline silicon solar battery, pyramid texture (alkali texture) is formed by wet etching using an alkaline aqueous solution such as NaOH or KOH and IPA (isopropyl alcohol). Although being effective for a case where a substrate is constituted by one crystal plane such as a monocrystalline silicon substrate because this technique uses the difference in etching rate on a crystal plane, this technique is unable to sufficiently reduce reflectivity for a case where various crystal planes are present in a plane of such a substrate as a polycrystalline silicon substrate.
To solve this problem, there is proposed a method in which a porous layer is formed on a surface of a silicon substrate by immersing the silicon substrate in a metal-ion (metal catalyst)-containing mixed aqueous solution of an oxidizing agent and hydrofluoric acid, and the silicon substrate is then immersed in an alkaline aqueous solution, thereby forming texture (see, for example, Patent Literature 1). However, the method of Patent Literature 1 has a problem that it is difficult to obtain stable solar battery characteristics. As measures to solve this problem, there is proposed a method in which a silicon substrate on which a porous layer is formed by a metal-ion (metal-catalyst)-containing mixed aqueous solution of an oxidizing agent and hydrofluoric acid is immersed in mixed acid of hydrofluoric acid and nitric acid, thereby forming texture (see, for example, Patent Literature 2 and Non Patent Literature 1).
According to the methods of Patent Literature 2 and Non Patent Literature 1, metal precipitated on the silicon surface forms pores in the silicon substrate as if the metal acts like a drill, and the precipitated metal is present on bottoms of the pores. Because mixed acid that is a mixture of the hydrofluoric acid and the nitric acid is an isotropic etchant for silicon, a pore diameter can be increased uniformly in a depth direction and the precipitated metal on the bottoms of the pores can be partially removed.
Furthermore, Patent Literature 3 describes that impurities such as metal impurities on a surface of a silicon substrate can be removed by a combination and repetition of ozone water, washing, and a hydrofluoric acid treatment. Patent Literature 4 describes a substrate cleaning method of rinsing a surface of a substrate with ozone water after performing a chemical treatment on the surface of the substrate and further rising the surface of the substrate with pure water. Patent Literature 5 describes a method of removing metal impurities adhering to a surface of a silicon wafer by rinsing the silicon wafer with ozone water.